I remember in cAnal when you added a new domain to your account, if setup properly it would automatically add a webmail.your.tld subdomain and would also allow your.tld/webmail to access webmail options.

I too have been looking for the best way to do this and I am by no means an expert (like Till or Falko , One Day!) but I have found a solution that worked for me.

The following will allow all domains, subdomains, and alias domains to access webmail using (as an example) http://your.tld/webmail

NOTE: I picked this up from some other post after hours of searching and did not bookmark the page but kept notes.

Step one: Make your symlink for webmail (taken from Step 17 of that link)

Code:

ln -s /usr/share/squirrelmail/ /var/www/webmail

Step two: Add global link for all sites
I use nano but you can use what you want:

This will work with a successful install of ISPConfig running on Debian.
Also if you have installed using the recommended install docs on the ISPConfig website you could choose to install SquirrelMail differently by changing the folder in Step one.
By doing so you can ultimately setup your /webmail to provide more than one webmail client.
EX: here

If you checked out the example you can see I offer more than just SquirrelMail but Horde and Roundcube.

Here is the concept:
If you follow Falko's "Perfect Setup guide" you will get to Step one (depending on your install) from the above post.

Insted of:

Code:

ln -s /usr/share/squirrelmail/ /var/www/webmail

Do

Code:

ln -s /usr/share/squirrelmail/ /var/www/webmail/squirrelmail

In doing so you link squirrelmail to /webmail/squirrelmail allowing yourself to add more than one webmail client to your /webmail link.
With some simple code you can create an index page that can point to your different webmail clients.